The north polar lakes of Titan as observed by Cassini Radar
Résumé
Over the course of a year, Cassini RADAR obtained Synthetic Aperture Radar images covering 69 percent of Titan's polar region north of 65 degrees; the region being 1.4E6 km3 in extent, greater than double the land area of the USA. We observe several hundred lakes with a range of morphological expression, including areally massive and morphologically distinctive "seas", covering ~15% of the polar region. Lakes are extremely radar dark, consistent with a lossy liquid hydrocarbon. Preliminary laboratory estimates suggest that loss tangents in the range 10E4 to 2x10E3 are reasonable, which implies that one can see through at least a few to many tens of m of liquids before the noise floor is reached, consistent with observed brightening towards many lake shores. North polar lake volumes are most likely in the 8E3 - 1.4E6 km3 range. Uncertainties will be reduced as more data, both image-based and experimental, are obtained but we can conclude with a high degree of confidence that hydrocarbon lakes on Titan are more voluminous than known terrestrial oil reserves; current estimates range from 2248 - 3896 billion barrels of oil (J. Hakes, 2000, Long Term World Oil Supply, Meeting of the Am. Ass. Pet. Geol., 18th April 2000, New Orleans, LA, http://www.eia.doe.gov/pub/oil_gas/petroleum/presentations/2000/long_term_supply.), hence 357 - 619 km3 . Small lakes often occupy steep-sided depressions, and although thermal and cryovolcanic origins cannot be completely ruled out, we are seeing growing geomorphologic evidence for dissolution chemistry, indicative of karst-like geology. The dichotomy between small lakes over slightly more than one half of the region, and seas on the other half, may be best explained by a topographic anomaly causing sub-surface flow of materials from the lakes to the seas. This may also explain observations by the Cassini ISS team (E. Turtle et al., in prep.) of a putative massive sea extending considerably further south than other observed north polar lakes.